1. Field of the Invention
This invention relates to a method and apparatus for transmitting light energy through a light energy absorbing medium. In one aspect, this invention relates to the transmission of laser energy through a light energy absorbing medium. In another aspect, this invention relates to the transmission of laser energy through a light energy absorbing material to a target area in a subterranean wellbore.
2. Description of Related Art
It is well known that substantial, heretofore untapped reserves of fossil fuels, including oil and natural gas, are buried deep within the ground. Access to these reserves may be obtained by means of wellbores that are produced by drilling apparatuses, which generally employ drill bits having hard and durable cutting contact elements. However, frequently these reserves are disposed beneath hard geological formations, e.g. rocks, which may even be impenetrable using conventional drilling equipment. Drilling of such hard formations requires a substantial amount of time and generally results in high costs. In addition, the drill bits used to drill through these hard formations are subject to extensive wear and/or damage. Thus, impenetrable formations may require the use of overly complex drilling routes in order to circumvent the formations and, in some cases, may result in complete abandonment of the drilling operation.
In addition to hard geological formations, unstable formations, e.g. shale, are frequently encountered, which formations may cause damage to and/or loss of drilling equipment. Unstable formations may also cause entrapment and subsequent abandonment of the drilling equipment.
One solution to the problems associated with the use of conventional drilling equipment, which as previously stated employ drill bits comprising one or more mechanical cutting elements, has been to use laser beams as a means of boring wells into the earth. For example, U.S. Pat. No. 4,066,138 to Salisbury et al. teaches an earth boring apparatus mounted above ground that directs an annulus of high powered laser energy downwardly for boring a cylindrical hole by fusing successive annular regions of the stratum to be penetrated at a power level that shatters and self-ejects successive cores from the hole. U.S. Pat. No. 4,113,036 to Stout teaches a laser drilling method and system of fossil fuel recovery in which a vertical bore hole is drilled into an underground formation, a laser beam is projected through the vertical borehole and reflected horizontally from the hole through the formation along a matrix of bores. U.S. Pat. No. 3,871,485 to Keenan, Jr. teaches a method of drilling using a laser beam in which a laser beam generator that is electrically connected to an inhole voltage generator actuated by drilling mud or other liquid passing through a laser beam housing connected to the drill string is positioned in the wellhole and a reflecting crystal is positioned within the laser beam housing to reflect the beam in an elliptical pattern across the formation to be penetrated. U.S. Pat. No. 5,107,936 to Foppe teaches a heat drilling process employing laser beams as a heat source in which the profile of the borehole is melted down by the heat source and the resulting molten rock is pressed into the surrounding side rock during the drilling process such that only a gap defining the outer profile of the borehole is melted down, which surrounds a drill core, which is extracted at an adjustable distance behind the melting zone.
It will be appreciated that the downhole environment of a wellbore during the drilling process as well as during other activities typically utilizing drilling, such as wellbore completion, the process of perforating the wellbore wall to initiate or enhance fluid flow into the wellbore, is extremely harsh, what with high temperatures and pressures, wellbore fluids, and debris generated during the drilling operation, and that such an environment presents substantial challenges to the effective use of lasers for drilling. One of the problems associated with the use of lasers in subterranean wellbores is the undesirable dissipation of the laser energy due to the debris and other environmental impediments encountered downhole. U.S. Pat. No. 6,888,097 to Batarseh teaches the use of a plurality of nozzles disposed around a laser head assembly which provide a purging gas for the purpose of removing dust or other particles from the exterior surface of transparent housing of the assembly. Suitable purging fluids may be gas, such as high pressure air, or liquids. Also taught therein is the use of at least a portion of the nozzles as vacuum nozzles connected to a vacuum source and adapted to remove gaseous fluids from around the exterior of transparent housing. While suitable for use in addressing the removal of dust and other particles, the apparatus does nothing to address the dissipation of laser energy resulting from the presence of fluids which may be present downhole, such as dirty water, drilling muds, and other wellbore fluids. Thus, there is a need for a method and apparatus by which the dissipation of laser energy in the downhole environment may be substantially reduced.
U.S. Pat. No. 4,090,572 to Welch teaches a method and apparatus for laser treatment of geological formations in which a laser beam is projected into a wellbore along a tubular beam guide so as to provide sufficient laser energy to melt or vaporize the formations under down-hole conditions. In this way, the laser beam is isolated from the surrounding fluids by a solid tubular beam guide, albeit at the expense of an additional amount of downhole equipment. U.S. Pat. No. 5,570,447 to Liu teaches the use of water or other aqueous liquid disposed in a rigid capillary or other suitably shaped inflexible vessel as a light conducting core medium of an elongated, small diameter vessel employed for light transmission suitable for spectrometry, photometry, and fluorimetry. See also U.S. Pat. No. 3,894,788, which teaches a liquid-core fiber-optic waveguide comprising a hollow vitreous fiber filled with a liquid having a refractive index greater than that of the vitreous fiber. It will be appreciated that in each instance of light transmission in accordance with the teachings of the prior art the light beam is isolated from the surrounding environment by some form of containment vessel. It will also be appreciated that the space available for downhole operations is limited, making it desirable to limit the amount of additional equipment employed to address the problem of laser energy dissipation.